Microelectromechanical systems (MEMS) refer to a collection of micro-sensors and actuators, which can react to an environmental change under micro-circuit control. The integration of MEMS into traditional radio frequency (RF) circuits has resulted in systems with superior performance levels and lower manufacturing costs. The incorporation of MEMS based fabrication technologies into micro and millimetre wave systems offers viable routes to devices with MEMS actuators, antennas, switches and capacitors. The resultant systems operate with an increased bandwidth and increased radiation efficiency, reduced power consumption, and have considerable scope for implementation within the expanding area of wireless personal communication devices.
MEMS elements comprise a first and a second electrode of which the second electrode is movable to and from the first electrode between a first position and a second position. In the first position the MEMS element is opened and there is a gap, usually an air gap, between the first and the second electrode. In the second position, the MEMS element may be closed, such that there is no gap between the first and the second electrode, or the airgap has a minimum thickness. Such minimum thickness can be achieved with the provision of bumps with the desired thickness. A dielectric layer may be present on top of the first electrode, that is generally present on a substrate. This leads thereto that the first electrode does not make electrical contact with the second electrode in its closed position, but forms a capacitor therewith. The other electrode or electrodes may also be provided with dielectric layers or native oxides if so desired.
The fact that the second electrode must be movable, but still be incorporated in a mechanically stable construction results therein that the devices are usually provided with a mechanical layer of sufficient thickness and mechanical stability. This layer may be a metal layer, but is alternatively a piezoelectric layer, such as is known from GB-A 2,353,410.
A device as described in the opening paragraph is known from WO-A 2004/54088, particularly FIG. 11 thereof. Both the intermediate layer and the mechanical layer are herein made of aluminium. These layers are mutually attached by a vertical interconnect with a diameter small than that of the second electrode in the intermediate layer.
It is a disadvantage of the known device that the capacitance density in a second position is less than was expected on the basis of the available surface area. Hence, the effective tuning range is also reduced, and with the resulting reduced tuning range the MEMS element is not very competitive to alternative solutions for tunable capacitors and switches. Such an alternative is in particular the use of discrete switches like pindiodes and pHEMT transistors, if needed in combination with discrete or thin-film capacitors.